Dosimetric and clinical validation of a high‐density bolus for superficial skin tumors treated with radiotherapy

Abstract

Abstract Background Accurate dose deposition in superficial radiotherapy remains challenging due to the inherent skin‐sparing effect of megavoltage photon beams and difficulties in ensuring bolus conformity. Purpose This study aims to comprehensively evaluate the dosimetric characteristics of the room‐temperature‐malleable high‐density bolus, to evaluate the accuracy of dose calculation and to assess clinical outcomes in a clinical cohort of 55 patients. Materials and Methods The dosimetric validation was performed through the use of dose measurements in a solid water phantom on top of which a 1.0 cm thick high‐densitybolus was positioned. Absolute depth dose measurements beyond the bolus were obtained by combining an absolute point dose (ion chamber) measurement at 1.0 cm depth beyond the bolus with relative depth dose measurements by using a microDiamond detector. Measurements were performed using 6 megavoltage (MV) flattening‐filter‐free (FFF) photon beams on an Halcyon and on a TrueBeam treatment unit. Obtained data were compared to dose calculations with both photon dose calculations available in the Eclipse treatment planning system (TPS): Analytical Anisotropic Algorithm (AAA) and Acuros XB (AXB). The impact of non‐conforming interfaces was assessed by introducing 0.5 to 2.0 cm thick air cavities between bolus and skin. Skin dose loss due to the air cavities was measured for static open fields and for dynamic sweeping gap fields. Clinical outcomes of 55 patients treated between October 2023 and March 2025 were retrospectively analyzed. Results Depth dose measurements confirm that a 1.0 cm high‐density bolus was sufficient to effectively overcome the build‐up region of the 6FFF photon beam. The AXB dose calculation algorithm provided the best agreement between measurements and calculations. Air gaps between the bolus and the skin resulted in clinically relevant skin dose reductions, ranging from approximately 1% to 20%, depending on field size and gap thickness, especially for clinical target volumes < 5.0 cm, large (> 1.0 cm) air gaps, or the use of large static fields. Among the 55 included patients, acute dermatitis toxicity was predominantly Grade 1 (49%) or Grade 2 (47.0%), with a low incidence of severe adverse events (Grade III: 4 %, Grade IV: 0%). Conclusion The high‐density bolus demonstrated satisfying dosimetric reliability, particularly when modeled with the Acuros XB algorithm. The bolus addressed the build‐up challenge inherent to MV photons, even in the presence of small air gaps, without increased toxicity in a large cohort of patients. This strategy represents a viable and accessible solution for ensuring accurate and conformal dose delivery in superficial tumors.